User Manual

Two Channel UHF Cell Enhancer
User Handbook
Handbook Number: 50-187601HBKM Page: 38 of 51
4.9 Channel Selective Module (17-003007)
4.9.1 Description
The channel selectivity module is employed when the Cell Enhancer requirement dictates that very
narrow bandwidths (single operating channels), must be selected from within the operating passband.
One channel selectivity module is required for each channel.
The Channel Selectivity Module is an Up/Down frequency converter that mixes the incoming channel
frequency with a synthesised local oscillator, so that it is down-converted to an Intermediate
Frequency (IF) in the upper HF range. An eight pole crystal filter in the IF amplifier provides the
required selectivity to define the operating passband of the Cell Enhancer to a single PMR channel.
The same local oscillator then converts the selected IF signal back to the channel frequency.
Selectivity is obtained from a fixed bandwidth block filter operating at an intermediate frequency (IF) in
the low VHF range. This filter may be internal to the channel selectivity module (Crystal or SAW filter)
or an externally mounted bandpass filter, (LC or Helical Resonator). Various IF bandwidths can
therefore be accommodated. A synthesized Local Oscillator is employed in conjunction with high
performance frequency mixers, to translate between the signal frequency and IF.
The operating frequency of each channel selectivity module is set by the programming of channel
selectivity module frequencies and is achieved digitally, via hard wired links, banks of DIP switches, or
via an onboard RS232 control module, providing the ability to remotely set channel frequencies.
Automatic Level Control (ALC) is provided within each channel selectivity module such that the output
level is held constant for high level input signals. This feature prevents saturation of the output mixer
and of the associated amplifiers.
Alarms within the module inhibit the channel if the synthesised frequency is not locked. The
synthesiser will not usually go out of lock unless a frequency far out of band is programmed.
The channel selectivity module is extremely complex and, with the exception of channel frequency
programming within the design bandwidth, it cannot be adjusted or repaired without extensive
laboratory facilities and the necessary specialised personnel. If a fault is suspected with any channel
selectivity module it should be tested by substitution and the complete, suspect module should then
be returned to AFL for investigation.